Listening to the song would have added it to your library. Think of your library as pretty much every song last.fm has counted as a "listen" or "scrobble".
How these scrobbles are counted depends on which client you use to listen to them with. For example, the last.fm desktop client has a setting that lets you decide how much of a song you need to listen to ...

I played around with books.google.com. It works great for keeping an online library. (You can find my personal library here.) There is also the Amazon Reading List Plugin for LinkedIn If you're a big LinkedIn user.